As a heat exchanger constructing a refrigeration cycle apparatus, there exists a heat exchanger including a heat transfer pipe having a circular shape. A diameter of the heat transfer pipe is progressively reduced for the purpose of achieving higher performance of the heat exchanger. In recent years, there even exists a heat exchanger including a flat perforated pipe used as the heat transfer pipe.
When a small-diameter circular pipe, for example, having a diameter of 4 mm or the like, or a flat perforated pipe is used as the heat transfer pipe, a flow passage sectional area of the small-diameter circular pipe or the flat perforated pipe is smaller than a flow passage sectional area of a normal circular pipe. Therefore, when the heat exchanger is formed with the number of passes equal to that in a mode in which the heat transfer pipe being a normal circular pipe is used, a pressure loss inside the heat transfer pipe is increased to lower operation efficiency of a refrigeration cycle.
Reduction of the pressure loss can be achieved by increasing the number of passes of the heat exchanger or reducing a length of the heat transfer pipe for one pass. When, for example, a related-art heat exchanger disclosed in Patent Literature 1 is operated as a condenser, in a main heat exchanger installed in an upper part, after a refrigerant is multi-branched at a header, the refrigerants are caused to flow in parallel. The refrigerants are condensed to cause a phase change from a gas refrigerant into a two-phase refrigerant having a large ratio of a liquid phase. After the refrigerants are re-joined together at a return header on an opposite side, the number of passes is reduced, and a flow rate is increased in a sub-heat exchanger installed in a lower part. Then, subcooling processing from the two-phase refrigerant into a liquid refrigerant is performed. Meanwhile, when the heat exchanger is used as an evaporator, the refrigerant flows from the sub-heat exchanger, and the two-phase refrigerant is evaporated into the gas refrigerant in the main heat exchanger. The sub-heat exchanger has a small number of passes. Thus, the pressure loss is large, and the amount of heat exchange with air is small. However, the sub-heat exchanger can increase a temperature of the refrigerant. As a result, condensed water remaining in the lower part can be prevented from turning into robust ice gorge (root ice) to break the heat transfer pipe or a fin.